1. Field of the Invention
The present invention relates to a bicycle and more particularly to such a front frame for a bicycle, which has strong structure and light weight.
2. Description of the Related Art
A conventional frame for a bicycle is made of metal and has strong structure and heavy weight. Recently, the bicycle is developed towards reducing its weight and adopts fiber reinforced material. As shown in FIG. 1, the frame 1 is made of carbon fiber reinforced resin and assembled by a front frame 2, a center shaft 3 and a rear frame 4. The front frame 2 has an upper tube 5, a head tube 6 and a lower tube 7. As shown in FIG. 2, the front frame 2 is manufactured by the steps of winding multiple layers of pre-pregs into an elongated tube 8, forming two openings 81 at two predetermined positions of the elongated tube 8, inserting a cured tube 9 into the elongated tube 8 through one of the openings 81 such that the two distal ends of the cured tube 9 are flush with the two openings 81 of the elongated tube 8, and placing the elongated tube 8 and the cured tube 9 into a die, and then curing the elongated tube 8.
The front frame 2 manufactured by aforesaid method has low structure strength since it is easy to crack at the junction of the cured tube 9 and the head tube 6. The upper tube 5 and the lower tube 7, which has not enough structure strength, are easy to break upon bearing large stress. If the quantity of the pre-pregs used in the front frame 2 is increase for enhancing the structure strength, the weight of the product is unintentionally increased. Therefore, there is always a problem as how to reduce the weight of the front frame 2 and increase the structure strength at the same time.